Reference is now made to copending patent application Ser. No. 207,748, for "Operator Programmable Inspection Apparatus," filed Nov. 17, 1980, and assigned to Inspection Technology, Inc., of Akron, Ohio, the assignee of the instant invention. This copending patent application presents in detail a system including a vidicon which is used to view workpieces, for example on an assembly line, and to generate a composite output signal of the RS-170 type. This signal is digitized and then compared against a stored digital image corresponding to a workpiece of known acceptable quality. Comparison is made by a digital processor under program control, and a determination is then made as to the acceptability of the workpiece based on the comparison.
It will be understood that many systems utilizing the teachings of the aforementioned copending patent application require a plurality of vidicons for simultaneous inspection of a number of areas on a large workpiece. It will also be appreciated that such vidicons, while being attractive from the standpoint of price, are given to problems of instability, distortion, and both physical and gray scale drift of the image with time, temperature, and the like. Present day solid state cameras are capable of overcoming certain of these problems, but the cost of such cameras is on the order of ten to thirty times that of a corresponding industrial grade vidicon. Additionally, solid state cameras are known to have typically slow scan cycles, low sensitivity, gray scale drift, and the inherent problem of "streaking." Of course, as is well known to those skilled in the art, the distortion experienced by a typical vidicon is either "pin cushion" or "barrel" distortion, most typically occurring at the edges or fringes of the video frame. While this particular type of distortion is not of grave importance, if the inspection apparatus is to be of optimum geometric accuracy, compensation must be made.
Applicants are unaware of any inexpensive and yet successful attempts in the art to overcome the foregoing problems which are experienced in the use of vidicons for inspection apparatus. Typically, the need for sensitivity or resolution in the system has not required such compensation. However, optimization of video inspection techniques requires that either expensive solid state cameras be utilized, or that compensation be made for the problems experienced with vidicons. It will be appreciated that the term vidicon as used herein generically refers to cameras providing a composite electrical output signal corresponding to the video image being sensed.